Desert Plant Communities Research Articles

Fungal endophytes in aboveground tissues of desert plants: infrequent in culture, but highly diverse and distinctive symbionts.

In hot deserts, plants cope with aridity, high temperatures, and nutrient-poor soils with morphological and biochemical adaptations that encompass intimate microbial symbioses. Whereas the root microbiomes of arid-land plants have received increasing attention, factors influencing assemblages of symbionts in aboveground tissues have not been evaluated for many woody plants that flourish in desert environments. We evaluated the diversity, host affiliations, and distributions of endophytic fungi associated with photosynthetic tissues of desert trees and shrubs, focusing on nonsucculent woody plants in the species-rich Sonoran Desert. To inform our strength of inference, we evaluated the effects of two different nutrient media, incubation temperatures, and collection seasons on the apparent structure of endophyte assemblages. Analysis of >22,000 tissue segments revealed that endophytes were isolated four times more frequently from photosynthetic stems than leaves. Isolation frequency was lower than expected given the latitude of the study region and varied among species a function of sampling site and abiotic factors. However, endophytes were very species-rich and phylogenetically diverse, consistent with less arid sites of a similar latitudinal position. Community composition differed among host species, but not as a function of tissue type, sampling site, sampling month, or exposure. Estimates of abundance, diversity, and composition were not influenced by isolation medium or incubation temperature. Phylogenetic analyses of the most commonly isolated genus (Preussia) revealed multiple evolutionary origins of desert-plant endophytism and little phylogenetic structure with regard to seasonality, tissue preference, or optimal temperatures and nutrients for growth in vitro. Together, these results provide insight into endophytic symbioses in desert-plant communities and can be used to optimize strategies for capturing endophyte biodiversity at regional scales.

Indirect effects in a desert plant community: do nurse-plants mediate interactions between understorey species?

Indirect effects in a desert plant community: do nurse-plants mediate interactions between understorey species?

Disturbances by desert rodents are more strongly associated with spatial changes in soil texture than woody encroachment

Background and Aims Soil texture is an important determinant of ecosystem structure and productivity in drylands, and may influence animal foraging and, indirectly, plant community composition. Methods We measured the density and composition of surface disturbances (foraging pits) of small, soilforaging desert vertebrates in shrubland and grasslands, both with coarse- and fine-textured soils. We predicted that the density and functional significance of disturbances would be related more to differences in texture than shrub encroachment. Results Soil texture had a stronger influence on animal foraging sites than shrub encroachment. There were more disturbances, greater richness and abundance of trapped seed, and greater richness of germinating plants on coarse- than fine-textured soils. Pits in coarse soils trapped 50 % more litter than those in finer soils. Apart fromslightlymoresoilremoval andgreaterlittercapture inshrublandpits,therewerenoeffectsofencroachment. Conclusions Although the process of woody encroachment has been shown to have marked effects on some ecosystem properties, it is likely to have a more subordinate effect on surface disturbances and therefore their effects on desert plant communities than soil texture. Our results highlight the importance of animal activity in shaping desert plant communities, and potentially, in maintaining or reinforcing shrub dominant processes.

Legacies of Prehistoric Agricultural Practices Within Plant and Soil Properties Across an Arid Ecosystem

Closely integrated research between archaeologists and ecologists provides a long-term view of human land use that is rare in the ecological literature, allowing for investigation of activities that lead to enduring environmental outcomes. This extended temporal perspective is particularly important in aridlands where succession occurs slowly and ecosystem processes are mediated by abiotic, geomorphic factors. Numerous studies show that impacts from ancient human actions can persist, but few have explored the types of practices or mechanisms that lead to either transient or long-term environmental change. We compared plant and soil properties and processes from a range of landscape patch types in the Sonoran Desert of the US Southwest that supported different, well-documented prehistoric farming practices from AD 750–1300. Our results show that the types of ancient human activities that leave long-term ecological legacies in aridlands are those that fundamentally alter “slow variables” such as soil properties that regulate the timing and supply of water. Prehistoric Hohokam floodwater-irrigation practices, but not dryland farming techniques, substantially altered soil texture, which was strongly associated with desert plant community and functional composition. However, prehistoric agriculture did not consistently alter long-term nutrient availability and thus had no impact on “fast variables” such as production of seasonal annual plants that are restricted to periods of ample rainfall. In this arid ecosystem, the inverse texture model explained patterns in plant functional composition at large scales, but is less predictive of production of short-lived desert annuals that experience a more mesic precipitation regime.

Effects of Artificial Sand Fixing on Community Characteristics of a Rare Desert Shrub

Eremosparton songoricum (Fabaceae) is a rare, native, clonal small shrub of the deserts of central Asia. Although human activities have greatly fragmented the distribution of E. songoricum, it occurs in areas where artificial sand fixing (AS) has been implemented. We sought to explore whether AS promotes survival and growth of E. songoricum. In the Gurbantunggut Desert of northwestern China in June 2010, we established 10 plots in an area where sand fixing occurred (5-10 years previously) and 11 plots on original sand substrate on which some plants had settled without fixing sand. Sand fixing changed soil properties and biological characteristics in sand-fixed plots. The soil surface where sand fixing occurred was covered by algal crusts and some lichen, but not bare sand (BS). Soil nutrients; water content of deep soil (30-150 cm); overall plant and herbaceous species richness, diversity, abundance, and cover; above- and belowground biomass; and cover, biomass, and height of E. songoricum in the sand-fixed plots were significantly greater than in plots of BS. However, distribution of E. songoricum individuals in the 2 types of plots did not differ. Our results indicate AS may enhance survival of E. songoricum and increase the overall diversity and stability of the desert plant community. We suggest AS as a way to protect this rare desert plant in situ.

基于盐分梯度的荒漠植物多样性与群落、种间联接响应

PDF HTML阅读 XML下载 导出引用 引用提醒 基于盐分梯度的荒漠植物多样性与群落、种间联接响应 DOI: 10.5846/stxb201306071403 作者: 作者单位: 新疆大学资源与环境科学学院,新疆大学资源与环境科学学院,华东师范大学环境科学系,新疆大学资源与环境科学学院,新疆大学资源与环境科学学院,新疆大学资源与环境科学学院,新疆大学资源与环境科学学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学重点基金项目(41130531) Responses of desert plant diversity, community and interspecific association to soil salinity gradient Author: Affiliation: College of Resources and Environment Science,Xinjiang University,College of Resources and Environment Science,Xinjiang University,Department of Environment Science,East China Normal University Shanghai,College of Resources and Environment Science,Xinjiang University,College of Resources and Environment Science,Xinjiang University,College of Resources and Environment Science,Xinjiang University,College of Resources and Environment Science,Xinjiang University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤盐分是影响干旱区荒漠植物群落动态的决定因素之一。基于样方调查和不同土壤盐分梯度下植物多样性指数及群落与种间关联的计算结果,分析干旱区荒漠群落植物多样性、群落联接性和种间关联对土壤盐分梯度的响应动态。结果表明,在土壤盐含量为0.03%-0.55% (S1)、0.61%-1.24% (S2)和1.41%-1.79% (S3)的盐分梯度上,(1)随土壤盐含量升高,群落生活型结构改变,草本比例减少,乔木比例增加;(2)植物多样性指数随土壤盐分增加而下降,低盐分梯度下,二者极显著正相关(P<0.01),中盐梯度下二者间呈部分显著负相关(P<0.05),高盐梯度下则转为以正相关为主(P>0.05);(3)群落联接性随土壤盐分梯度转变,在0.61%-1.24%的中度盐含量时正联接性最强(VR=1.89),高盐度含量下群落转为不显著的负联接,稳定性降低;(4)沿盐分升高梯度,种间的负相关种对数增加,正相关种对数减少,种间关联(相关)强度提高,正负相关种对比(PNR)与多样性指数呈显著正相关(P<0.05)。综上可知,干旱区荒漠植物多样性在土壤盐含量达到1.41%-1.79%水平时总体显著降低;土壤盐分水平显著影响植物群落和种间联接性,种间互利性随盐分梯度增加下降,物种趋于独立分布,并最终导致荒漠植物多样性降低。 Abstract:Dynamic response of plant diversity, community and interspecific association along environmental gradients is one of the basic issues in community ecology and biodiversity science. Soil salinity is one of the most decisive factors that govern desert plant community dynamics. The Ebinur Lake wetland national nature reserve is located in typical arid desert area. Previous related studies about the reserve had point out that soil salinity influences the desert plant community characteristic significantly, but the specific role of soil salinity in plant community dynamics is still unknown. Based on this, the objective of this study is focused on the responses of desert plant diversity, community overall association and interspecific association to soil salinity gradient.In this study, altogether 74 sampling plots were investigated; including 22 plots for trees (10 m×10 m), 35 plots for shrubs (5 m×5 m) and 17 plots for herbs (1 m×1 m). At each plot, abundance, height and diameter at breast height of trees, abundance of shrubs and herbs were surveyed. According to soil salinity of the corresponding plot, all plots were classified to 3 salinity gradients by cluster analysis, with the salt contents of low (S1), middle (S2) and high (S3) defined as 0.03%-0.55% (S1), 0.61%-1.24% (S2) and 1.41%-1.79% (S3) respectively. A range of different diversity indices including Shannon-Wiener, Simpson, Pielou, Fisher-α of logarithmic series distribution and Margalef were calculated to examine the plant diversity under each salinity gradient. Variance ratio (VR), χ2 test and Spearman rank correlation were employed to quantitatively analyze the community overall association and interspecific association under different salinity gradient. VR value, proportion of species pairs with positive and negative correlation, Dice index and Spearman rank correlation coefficient were used in exploring response of overall association and interspecific association to different soil salinity. The results indicated that: 42 species (which belong to 39 Genus of 18 Family) were identified in the Ebinur Lake wetland national nature reserve. Along the soil salinity gradient (S1-S3), (1) Life form structure in community was changed, more specifically, proportion of herbaceous decreased while tree proportion increased as salinity increases, and shrubs proportion decreased slightly. (2) Plant diversity indices decreased with increased soil salinity, and showed a very significant positive correlation in low salinity (P<0.01), then the correlation turned into partially significant negative (P<0.05) in middle salinity and showed positive mainly in high salinity gradient (P>0.05). (3) Community overall associations was significant positive in low (VR=1.42) and middle (VR=1.89) salinity, but converted to negative (VR=0.62) in high salinity, which means that high soil salinity lead to the decrease of community stability. (4) Along rising gradient of soil salinity, number of species pairs with a negative correlation increased, while number of species pairs with a positive correlation decreased; both of positive and negative correlation strength (coefficients) was enhanced as soil salinity increases; ratio of positive species pairs to negative species pairs showed a significantly positive correlation with diversity (P<0.05). In conclusion, reduced desert plant diversity and shifted community and interspecific association under high salinity threatened plant community stability of arid area, and may lead to retrogressive succession of community. Results of the present study provide a scientific basis for arid area in vegetation management, vegetation restoration and soil salinization prevention. 参考文献 相似文献 引证文献

Variation in monsoon precipitation drives spatial and temporal patterns of Larrea tridentata growth in the Sonoran Desert

Summary 1. Broad-scale patterns of above-ground net primary production (ANPP) are closely coupled to climate features, particularly the distribution and magnitude of rainfall. In arid and semi-arid regions, however, the precipitation regime, together with local geomorphology and plant life history traits, combine to regulate soil water availability and patterns of growth, complicating simple correlations with climatic variables. 2. To better understand the drivers of plant growth in desert ecosystems, we characterized the rates and spatial heterogeneity of stem elongation by the dominant perennial shrub, creosote bush (Larrea tridentata) in the northern Sonoran Desert of Arizona (USA). Estimates of stem growth were made over a 5-year period (2006–2010) from 60 plots at 15 sites spanning c. 120 km across the Central Arizona–Phoenix (CAP) Long-term Ecological Research (LTER) area. 3. We observed both the highest and lowest rates of stem growth during summer, and these patterns were related to the amount of monsoon rainfall and local rates of water infiltration. The relationship between growth and precipitation in the summer was nonlinear, with rates increasing more than eightfold at plots receiving more than 100 mm of seasonal rainfall. Conversely, growth during the winter/spring was intermediate in magnitude, similar among years and poorly correlated with seasonal precipitation. 4. The spatial heterogeneity of stem growth also differed markedly between seasons and was greater both within and among sites during summer compared to winter/spring growing periods. At broad scales, spatial heterogeneity of shrub growth was correlated with seasonal changes in the spatial variability of rainfall across the study area. At small spatial scales, greater heterogeneity during the summer appears linked to local soil properties that influence infiltration and water availability following monsoon storms. 5. Overall, the strong, nonlinear growth response by L. tridentata to monsoon rainfall suggests that the recurrence interval of wet summer seasons is an important driver of ANPP for this long-lived shrub. More generally, our results illustrate how regional climate variability and local soil properties can interact to influence the rates and heterogeneity of desert plant growth at multiple scales.

Spatial and temporal structure of the spider community in the clay semi-desert of western Kazakhstan

The spatial and temporal structure of spider communities was studied in the clay semi-desert of the north-western Caspian Lowland, western Kazakhstan (49°23' N, 46°47' E). The soils and vegetation are complex, being composed of a mosaic of desert and steppe plant communities. Besides the native associations, there are plantations of different tree species. The ground-dwelling spider assemblages in the native habitats are the most diverse. The number of species inhabiting forest plantations is three times as small. Gnaphosidae is the leading family in the ground layer. They show high abundance and diversity levels during the whole season. Thomisidae, Lycosidae, Philodromidae, and Salticidae are abundant as well. The species diversity of herbage-dwelling spiders in different open native habitats is very similar. The spectrum of dominant families (Thomisidae, Oxyopidae, Araneidae, and Salticidae) and the seasonal dynamics of their ratio in desert and steppe associations have much in common. Spider assemblages of native and artificial habitats are characterised by change from multispecies polydominant spring-summer communities to impoverished imbalanced autumn ones. Seasonal changes in the species structure of mature spider groupings in native habitats are well pronounced, while the impact of seasonal conditions is even stronger than between-habitat differences. Complexes of typical species with different levels of habitat preference are revealed.

Resolution of Respect: Professor Paul Schultz Martin 1928–2010

Resolution of Respect: Professor Paul Schultz Martin 1928–2010

Revegetation of Degraded Winterfat Communities

Many of the shrubs growing in the salt desert environments have spines, modifi ed leaves that impede browsing animals like cattle. However, winterfat does not possess such deterrents. This silver-gray shrub is a compo-nent of a variety of salt desert plant communities in Nevada, including lake plains and alluvial fans where it often dominates extensive areas along with bud sage (

Trophic relationships and their importance for biotopic distribution of grasshoppers (Orthoptera, Acridoidea) in semi-deserts and deserts of the lower Volga river area

The trophic associations of grasshoppers in the semi-deserts and deserts of the lower Volga area are analyzed. Most of the species studied are narrow oligophages or monophages; they constitute four groups trophically associated with edificators of the commonest desert plant communities (hygro- and mesophytic grasses, xerophytic grasses, wormwoods, and chenopods). The trophic specialization is shown to contribute significantly to the formation of biotopic association and structure of grasshopper assemblages.

Study on the Phenological Characteristics of the Main Plant Species and the Use of Pastures in the Minqin Desert Areas

Research on grazing intensity is a hot field in grassland research in recent years.Is it possible that the moderate grazing results in a degeneration of desert plant communities? What is the basis for the use of moderate grazing and how can a rational grazing be implemented? In order to find the solutions of these issues,in this paper the phenological characteristics of the main plant species and the use of pastures in the Minqin desert areas are analyzed. A key factor restricting plant growth is analyzed using the relationship between vegetation status and precipitation. The availability and accessibility of the extent of desert steppes are researched using the data of vegetation and soil seed bank of the sand dunes with different stability levels.The main season of the use of shrubs and herbaceous plants is analyzed using the phenological data in the Minqin desert areas.The results show that plant seeds are rich in soil seed bank in the Minqin desert steppes.Precipitation is the key factor restricting plants density and vegetation coverage.It is possible to graze moderately in the steppes.It is necessary to choice the plant communities according to their leaf shooting stage and fruit-mature stage to graze by taking an intermittent rotation way.The first grazing season is from the beginning of leaf shooting stage,and the second one is from the beginning of fruitmature stage.

Animal disturbances promote shrub maintenance in a desertified grassland

Summary1. Soil disturbance by animals affects the availability of water, nutrients, sediment and seeds, which are critical for the maintenance of functional ecosystems. We examined long‐lived faunal structures across six vegetation communities in the northern Chihuahuan desert of New Mexico, USA, testing the proposition that disturbances in undesertified grassland differ in magnitude and effect from those in desertified grassland.2. Vertebrate and invertebrate disturbances totalled 18.9 structures ha−1across 18 sites. The most common were pits and mounds of American badgers (Taxidea taxus,32%), nests of the antAphaenogaster cockerelli(18.8%) and mounds of kangaroo rats (Dipodomys spectabilis, 31%).3. Desertification was associated with a doubling of the density of structures, but no effects on cover or volume. The greatest density was in desertified mesquite and creosote bush shrublands, and the lowest density in undesertified grass swales. Badger and wood rat (Neotomasp.) mounds were significant indicators of desertified communities.4. Desertification did not affect the density of kangaroo rat mounds (6.7 ha−1in black grama grasslands and creosote bush shrublands). However, mounds in creosote bush shrubland were smaller and had more and larger shrubs than adjacent inter‐mound hummocks. Desertification was associated with increases in the density ofAphaenogaster cockerelliandTrachymyrmex smithiinests, and declines inPogonomyrmex rugosusnests. Substantial increases in soil nitrate and electrical conductivity onMyrmecocystusnests were associated with desertification.5. Synthesis. Desertification shaped this desert environment in two main ways. First, while kangaroo rat mound density changed little with desertification, mounds in shrubland continued to enhance shrub persistence long after abandonment, reinforcing desertification processes. Second, marked changes in the density of nests of the key ant species altered the spatial distribution of soil nitrate and electrical conductivity, likely affecting soil fertility and the distribution of desert plants. Our results highlight the importance of animal activity in shaping desert plant communities, and in maintaining or reinforcing desertification processes.

Plant community characteristics and their relationships with climate in the Hexi Corridor region of northwestern China

Species, coverage, density, appearance, important values and above ground biomass of desert plant communities in the Hexi Corridor region in northwestern China were investigated. Data on the meteorological factors at the investigation sites were collected. Their relationships were statistically analyzed. The results show that the composition of plant species and total density in the Hexi Corridor region are positively correlated with elevation and longitude, but negatively with latitude. The relation decreases from the southeast to the northwest in this region. In the desert regions with an accumulated temperature of >10°C greater than 3200°C, the shrubs, including semi-shrubs, occupy dominant positions in the communities. Their importance values reach 100. When the accumulated temperature is about 3000°C, the communities are often in some type of transition, either with herbage being the dominant species, with some semishrubs or shrubs as companion species, or the dominant species being semi-shrubs with some perennial herbage as companion species. In desert communities with an annual precipitation of over 110 mm, the main life forms of plants are semi-shrubs or perennial herbs. When precipitation is less than 110 mm, the main life forms of the vegetation are dwarf shrub or annual herbs. In the extreme arid regions with annual precipitation of around 50 mm in the northwestern part of the Hexi Corridor, barely some annual herbage come to life during the rainy season. Desert ecosystems consist of sparse vegetation with small biomass. They change with climate factors and are especially sensitive to precipitation, which is particularly important for their conservation.

Shrub Invasion Decreases Diversity and Alters Community Stability in Northern Chihuahuan Desert Plant Communities

BackgroundGlobal climate change is rapidly altering species range distributions and interactions within communities. As ranges expand, invading species change interactions in communities which may reduce stability, a mechanism known to affect biodiversity. In aridland ecosystems worldwide, the range of native shrubs is expanding as they invade and replace native grassland vegetation with significant consequences for biodiversity and ecosystem functioning.MethodologyWe used two long-term data sets to determine the effects of shrub encroachment by Larrea tridentata on subdominant community composition and stability in formerly native perennial grassland dominated by Bouteloua eriopoda in New Mexico, USA.Principal FindingsOur results indicated that Larrea invasion decreased species richness during the last 100 years. We also found that over shorter temporal scales species-poor subdominant communities in areas invaded by Larrea were less stable (more variable in time) compared to species rich communities in grass-dominated vegetation. Compositional stability increased as cover of Bouteloua increased and decreased as cover of Larrea increased.SignificanceChanges in community stability due to altered interspecific interactions may be one mechanism by which biodiversity declines in grasslands following shrub invasion. As global warming increases, shrub encroachment into native grasslands worldwide will continue to alter species interactions and community stability both of which may lead to a decline in biodiversity.

Effect of Herbivory and Disturbances by Tuco-Tucos (Ctenomys Mendocinus) on a Plant Community in the Southern Puna Desert

ABSTRACTWe studied the differences between areas undisturbed and disturbed by Ctenomys in the structure, diversity, and composition of a Puna desert plant community. We also studied differences in nutrient distribution in the soil produced by the activity of tuco-tucos. Within the plant community, plant and soil samples were taken from different sites, and at each site, both disturbed and undisturbed areas were sampled. We hypothesized that the activity of this rodent affected plant community structure, specific composition, diversity, and nutrient distribution (N, K, and P). Results at the plant population and community level and for soil nutrient concentration suggest that Ctenomys mendocinus could be a keystone species, capable of orienting the dynamics of the plant community studied in this ecosystem. Nevertheless, further manipulative experiments are necessary to confirm that the differences found between disturbed and undisturbed areas are indeed caused by the activity of Ctenomys.

Elevated CO2 increases microbial carbon substrate use and nitrogen cycling in Mojave Desert soils

AbstractIdentifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. We assessed the effects of elevated atmospheric CO2 on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community‐level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata) during the 2004–2005 growing season, an above‐average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO2, and sampling date. Microbial C and N activities were lower in interspace soils compared with soils under Larrea, and responses to date and CO2 treatments were cover specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO2, but increased under elevated CO2. Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO2 positively affected both. Elevated CO2 also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils under Larrea, and was lower in soils exposed to elevated CO2. Gross rates of NH4+ transformations increased over the growing season, and late‐season NH4+ fluxes were negatively affected by elevated CO2. Gross NO3− fluxes decreased over time, with early season interspace soils positively affected by elevated CO2. General increases in microbial activities under elevated CO2 are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO2, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO2 shift the spatial distribution of Mojave Desert plant communities.

Diversity of arbuscular mycorrhizal fungi associated with desert ephemerals in plant communities of Junggar Basin, northwest China

Colonization by and diversity of arbuscular mycorrhizal (AM) fungi associated with five common ephemerals, Eremopyrum orientale (L.) Jaub. et Spach., Gagea sacculifera Regel., Plantago minuta Pall., Tragopogon kasahstanicus S. Nikit., and Trigonella arcuata C. A. Mey. were investigated in four typical desert plant communities in Junggar Basin, northwest China. All five ephemerals examined were found to be colonized and formed typical arbuscules or vesicles. The proportion of root length colonized ranged from 2 to 85% with an average of 19%. Spore density in soil near the roots of different ephemerals varied from 1 to 120 spores per 20 ml soil, with an average value of 33 spores. Species richness averaged 8.8 AM fungal species in soil near the roots and ranged from 2 to 21. Fifty-four AM fungal taxa belonging to the genera Acaulospora, Archaeospora, Entrophospora, Glomus and Paraglomus were isolated and identified from soil around the roots. Glomus was the dominant AM fungal genus with a frequency of 100% and relative abundance of 82.6%. The AM fungal species with the highest frequency of occurrence was Glomus aggregatum with a frequency of 75%. G. microaggregatum was present in the highest relative abundance (16%). G. sacculifera, P. minuta and T. arcuata formed Arum-type mycorrhizas. Arbuscular mycorrhizal fungi are ubiquitous and Arum-type mycorrhizas are especially prevalent in these important desert communities used for grazing and traditional medicine.

Incorporating Parametric Uncertainty in the Design of Alternative Landfill Covers in Arid Regions

Monte Carlo simulations and a combination of site‐specific data (e.g., soil properties, climatic conditions, and native vegetation) were used to design alternative (evapotranspiration) landfill covers at Edwards Air Force Base, located near Lancaster, CA. Laboratory analyses of site soils indicated the presence of three distinct surface soils, from which statistical distributions were generated. A 10‐yr climate sequence (precipitation and potential evapotranspiration) was used for the upper boundary. Potential evapotranspiration was partitioned into potential evaporation and potential transpiration using the phenology of a Mojave Desert plant community. Nearly 1000 realizations were run for each of 72 different combinations of soil type, cover thickness, and plant cover percentage. The results indicate that threshold design parameters, needed to limit deep flux to &lt;0.5 cm yr−1, differ based on the relationship between the Ks (saturated hydraulic conductivity) of the surface soil, cover thickness, and plant cover percentage. In the lower conductivity soils (mean Ks = 20 cm d−1), deep flux was ≤0.2 cm yr−1 for a cover thickness &gt;80 cm with a plant cover &gt;10%. Higher conductivity soils (Ks = 250 cm d−1) required thicker soils covers (&gt;100 cm) and greater plant cover (&gt;20%) to achieve similar fluxes. In all cases, variations in both cover thickness and plant cover percentage indicated threshold values, above which incremental additions added little to cover performance. The methods developed here could be implemented at other sites where conditions are known. Designs can account for uncertainties in site parameters and contribute to improved decision making.

Growth and water and nitrate uptake patterns of grazed and ungrazed desert shrubs growing over a nitrate contamination plume

Two native desert shrubs were evaluated for their growth potential and water and nitrogen uptake patterns over a nitrate-contaminated aquifer at a former uranium ore-processing facility in northeastern Arizona. Sarcobatus vermiculatus and Atriplex canescens are obligate and facultative phreatophytes, respectively, that dominate the local desert plant community. The main questions we addressed were: (1) Are these shrubs able to use water or nitrogen from the alluvial aquifer? (2) If so, does grazing interfere with that ability of shrubs? (3) What would be the ideal strategy to take up N from the plume and prevent its expansion and recharge using shrubs? δ 18O and δD isotope signatures from water in plant stem samples suggest that both species utilize mainly deep, stored soil water derived from winter rains for transpiration, rather than summer rains or plume water. δ 15N enrichment values were similar for leaves of plants growing on and off the plume and for soil and aquifer water samples, but nitrate-N levels in leaf tissues were five times higher in plants growing on the plume compared to off the plume, suggesting they may have derived at least part of their nitrogen from the contamination plume. Total leaf N was also higher for plants growing on the plume. Under present conditions, only about 5% of the area over the plume is vegetated. Plants protected from grazing inside exclosures increased in volume by 2–4-fold over three growing seasons. Transplants of A. canescens, protected from grazing and irrigated over the first summer, established readily and grew into large shrubs after 3 years. On the basis of this study, the shrub community could be increased to as high as 25% cover and could make a significant contribution to controlling recharge if the contaminated site was protected from grazing. The results suggest that deeply rooted desert shrubs can impact the subsoil water and nitrogen balance, and that this balance can be disrupted by land use practices such as overgrazing that degrade the vegetation cover.